The primary focus of this research program is on different aspects of genetic recombination, especially on the effects of specific gene mutations on meiotic gene conversion and mitotic crossing over. The experiments will be done wIth the yeast Schizosaccharomyces pombe. The program planned has grown out of an analysis of the mutant M26 in the gene ade6 of Schiz. pombe. M26 shows a pronounced marker effect with respect to gene conversion: in crosses of M26 with the wild type significantly more conversion tetrads occur than in crosses of other ade6 alleles with the wild type, and tetrads being 3 plus: 1 M26 are 12 times more frequent than 1 plus: 3 M26 tetrads. Our results can be explained by the hypothesis that single strand breaks frequently occur in the DNA at the mutation site of ade6-M26, which are the starting points for gene conversion events. If the M26 mutation is not present, such breaks do not occur frequently in this region of the gene. If this is true, M26 would be a mutant of great general interest for studies on the mechanism of recombination. The propoed research included expriments on the following topics. (1) Crosses which will test the hypothesis that single strand breaks occur preferentially at the M26 site. The crosses will be analyzed either by tetrad analyses or by plating of free ascospores. (2) M26 is a nonsense mutation. It is planned to test whether further mutational changes in this nonsense codon also affect the occurrence of gene conversion in this region of the DNA. (3) We have recently obtained recombinationless mutants. A detailed study of these mutants is planned, in particular as to their effect on M26. (49 In addition to the experiments relating to gene conversion, some studies are planned on mitotic crossing over and the mating-type system of Schiz. pombe.